Hmmm.... The only thing I can think of is the placement of the waypoint. Are you sure that you placed it on the truck? You have to select the soldiers, make sure your in waypoint mode, and actually double click on the truck to place the waypoint. It dosn't work if you place the waypoint, and then just drag it on top.
EDIT:
Or, just give the soldiers a "GET IN" waypoint and place it somewhere close to the truck. Then, give the truck a "LOAD" waypoint. Then snyc both waypoints.